1. Field of the Invention
The present invention relates to a lithium secondary battery.
2. Description of Related Art
Recently, demands, including reduction in size and higher energy density have been increasing on power sources for mobile communication devices such as cellular phones and portable personal computers. Meanwhile, midnight power storage systems and power storage systems involving solar cells and wind-power generation have also been increasingly developed. Electric vehicles, hybrid vehicles, and hybrid electric trains using electric power for partial motive energy have been increasingly available for practical use.
However, nonaqueous electrolyte lithium secondary batteries utilizing carbonaceous materials, silicon materials, metal oxides, and the like as negative electrode materials are problematic in that: the organic solvent included in the electrolyte is reductively decomposed on the negative electrode surface in a charge-discharge process; and the negative electrode impedance is increased over time due to the gas generation and the deposition of the reductively decomposed substances of the organic solvent, resulting in the battery capacity degradation.
Accordingly, for the purpose of suppressing the above described reductive decomposition of the organic solvent, various compounds have hitherto been added to the electrolyte as means for suppressing the reductive decomposition of the organic solvent on the negative electrode. Hence, techniques to control the morphology of the negative electrode surface coating have become significant. For example, JP Patent Publication (Kokai) No. 2001-057234 A, JP Patent Publication (Kokai) No. 2004-154352 A, and JP Patent Publication (Kokai) No. 2004-022379 A disclose the addition of vinylene carbonate, a pyridine derivative, and lithium difluoroacetate to the electrolyte, respectively. However, even the use of these compounds as additives results in battery storage properties that are unable to cope with recent demands for higher capacity.